Wildfires can pose unique challenges for even well-trained firefighters, as the latest inferno in Yarnell proved.

Each year, the Centers for Disease Control (CDC) estimates that hundreds of thousands of acres burn across the U.S., and on Sunday, a perfect storm of dry conditions, high winds and lightning torched 8000 acres in Arizona, claiming the lives of 19 firefighters who were trapped while trying to control the blaze. According to the Associated Press, the fire has destroyed 200 homes.

Experts say that wildfires can be doubly dangerous for firefighters; when homes and forest burn simultaneously, both heat and smoke conspire to make air unbreathable. Below are a few of the risks wildfire firefighters like the elite Hotshot crew face in the line of duty:

Heat: In extremely high temperatures and wearing heavy protective gear, firefighters often experience heat stress, which can lead to nausea, light-headedness and weakness. This can progress into heat strain, which causes both the core body temperature and heart rate to soar. If there’s no relief from the heat, a fire fighter can experience heat stroke. “The ambient temperature is in the hundreds of degrees and you have protective gear on top of that,” says Lou Paulson, president of California Professional Firefighters. “Plus you are doing arduous tasks so you have an elevated heart rate for a long period of time.”

Preliminary reports suggest that the Hotshot firefighters were trapped when a sudden shift in wind pushed the blaze toward them, forcing them to take cover under makeshift fire-resistant tents. The shelters, however, provide at best 10 minutes of protection under moderately intense conditions — and the Yarnell fires likely reached triple digits in temperature.

Toxic Air: Smoke and dust from a wildfire produce a variety of toxic substances and gases like carbon monoxide, sulfur dioxide and formaldehyde. “The problem with wild firefighting is that unlike structure fires where you have a self contained breathing apparatus, in wild firefighting there is no way you will have an air supply that will last long enough to do that,” says Paulson. “A filter mask has to be filtered for the particulates that you are dealing with, which you don’t know because you are in a wildland environment.”

That is why smoke inhalation is one of the leading causes of death from a fire, and for longtime blaze battlers, the effects can become chronic. “You can make a strong analogy to people who smoke cigarettes. You get tars and all those things for a long period of time. Being exposed to smoke with no respiratory protection becomes a huge problem. It becomes a huge risk as it relates to your cardiopulmonary functions and potentially cancers,” says Paulson.

Fatigue: During the initial surge of the fire, the front line is very difficult to control, according to Paulson. “You can have folks on the fireline for 72 hours or longer with no break at all. There are long work periods and no rest,” he says. It’s not uncommon that firefighters get so exhausted that they succumb to an intense need to nap on the fire line after long hours fighting relentless flames and inhaling toxic gases.

The irregular and lengthy work hours only add to the fatigue, since crews are frequently forced to work on little sleep. The CDC reports that that working 12 hours or more a day is associated with a 37% increased risk of injury.

According to the Wall Street Journal, firefighting agencies have improved their emergency management strategies, including providing elite crews like the Hotoshots with better ways to shelter in place when flames make it impossible for them to outrun a wildfire. Fires, however, are notoriously unpredictable, and large fires like the one in Yarnell can generate their own weather systems, when clouds that form from the water in heavy smoke break apart, sending gusts of wind in all directions. But when the flames and smoke consume all of the available air, no amount of training can compensate for what the lungs and tissues so desperately need.